Abstract. This paper presents an evaluation of a new linear parameterization valid for
the troposphere and the stratosphere, based on a first order approximation of
the carbon monoxide (CO) continuity equation. This linear scheme (hereinafter
noted LINCO) has been implemented in the 3-D Chemical Transport Model (CTM)
MOCAGE (MOdèle de Chimie Atmospherique Grande Echelle). First, a one
and a half years of LINCO simulation has been compared to output obtained
from a detailed chemical scheme output. The mean differences between both
schemes are about ±25 ppbv (part per billion by volume) or 15% in the
troposphere and ±10 ppbv or 100% in the stratosphere. Second, LINCO has
been compared to diverse observations from satellite instruments covering the
troposphere (Measurements Of Pollution In The Troposphere: MOPITT) and the
stratosphere (Microwave Limb Sounder: MLS) and also from aircraft
(Measurements of ozone and water vapour by Airbus in-service aircraft: MOZAIC
programme) mostly flying in the upper troposphere and lower stratosphere
(UTLS). In the troposphere, the LINCO seasonal variations as well as the
vertical and horizontal distributions are quite close to MOPITT CO
observations. However, a bias of ~−40 ppbv is observed at 700 Pa
between LINCO and MOPITT. In the stratosphere, MLS and LINCO present similar
large-scale patterns, except over the poles where the CO concentration is
underestimated by the model. In the UTLS, LINCO presents small biases less
than 2% compared to independent MOZAIC profiles. Third, we assimilated
MOPITT CO using a variational 3D-FGAT (First Guess at Appropriate Time)
method in conjunction with MOCAGE for a long run of one and a half years. The
data assimilation greatly improves the vertical CO distribution in the
troposphere from 700 to 350 hPa compared to independent MOZAIC profiles. At
146 hPa, the assimilated CO distribution is also improved compared to MLS
observations by reducing the bias up to a factor of 2 in the tropics. This
study confirms that the linear scheme is able to simulate reasonably well the
CO distribution in the troposphere and in the lower stratosphere. Therefore,
the low computing cost of the linear scheme opens new perspectives to make
free runs and CO data assimilation runs at high resolution and over periods
of several years.